Abstract

Cascade conical refraction occurs when a beam of light travels through two or more biaxial crystals arranged in series. The output beam can be altered by varying the relative azimuthal orientation of the two biaxial crystals. For two identical crystals, in general the output beam comprises a ring beam with a spot at its centre. The relative intensities of the spot and ring can be controlled by varying the azimuthal angle between the refracted cones formed in each crystal. We have used this beam arrangement to trap one microsphere within the central spot and a second microsphere on the ring. Using linearly polarized light, we can rotate the microsphere on the ring with respect to the central sphere. Finally, using a half wave-plate between the two crystals, we can create a unique beam profile that has two intensity peaks on the ring, and thereby trap two microspheres on diametrically opposite points on the ring and rotate them around the central sphere. Such a versatile optical trap should find application in optical trapping setups.

Figures (8)

Schematic of conical refraction. A beam traversing a crystal of length (L) is refracted through a semi-angle (A) to give a ring radius of R0. The beam shift direction (γ) is in the direction of the parallel polarization component towards the orthogonally polarized component at the opposite side of the ring.

Experimental setup for observation of focal image plane profiles. The two biaxial crystals are located between the two lenses, one rotated at an angle α around the beam axis; optical elements can be placed after the laser or in between the crystals and include linear polarizer, quarter or half wave-plate, or some combination of these.

CCD images of the FIP with (a) α = 0°,(b) α = 20°, (c) α = 45°, (d) α = 90°. (e) Processed image of all frames from α = 0 to 180°. The path of the Gaussian central spot travels along the patch of the ring generated from the first crystal and increases in intensity. is

Several individual frames showing how a particle trapped in the crescent beam orbits around the particle trapped in central beam spot. Media 1 shows the rotation of the outer microsphere with respect to the centrally trap microsphere as the plane of polarization of the incident beam is rotated.

Frames from rotation of two diametrically opposed particles trapped in lobes around a stationary particle trapped in the centre. Media 2 shows the continuous anti-clockwise rotation of the two microparticles around the central particle trapped in the Gaussian spot.